LIGHT AND VEGETATION 



23 



The absorption of infra-red by water is a property often 

 used in the laboratory for eUminating these wave-lengths 

 from the light emitted by incandescent sources. The passage 

 through a solution of copper sulphate, or better still through 

 a 2 per cent solution of cupric chloride 2 cm. thick, is a 

 more effective means of almost completely eUminating the 

 invisible rays of long wave-lengths (see Fig. I, 3). 



Glass absorbs all wave-lengths less than about 3,500 A. 

 Thus, in front of a lamp emitting ultra-violet, our eyes are 

 protected by a thin sheet of glass or by ordinary glass lenses. 

 Plants cultivated in a glasshouse never receive the photons of 



Visible Infra-red 



Fig. 1, 4. Transmission curve of three coloured glasses. Note 

 their strong transmission in the near infra-red, particularly 

 that of the blue glass (which is unexpected) 



the sun of wave-lengths between 3,600 and 2,900 A, with 

 which plants in the open air are well provided. 



Coloured filters, whether they are made of glass or of 

 gelatine containing certain pigments, or whether they consist 

 of liquids contained in vessels of glass or quartz, permit the 

 passage of large bands of wave-lengths of which the limits 

 are not very clearly defined. The properties of a filter are 

 described entirely by its transmission factor for each wave- 

 length; Fig. I, 4 gives some examples of the transparency curves 

 of common coloured glasses. 



Some large firms give, in their catalogues, Usts of coloured 

 glasses with their transmission curve] these glasses vary in 

 price and durability. 



